linux-kernel - Re: [PATCH v2] ASoC: tas5805m: fix pdn polarity

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Date:   Thu, 10 Mar 2022 20:09:42 +0000
From:   John Keeping <john@...anate.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     alsa-devel@...a-project.org, Liam Girdwood <lgirdwood@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Daniel Beer <daniel.beer@...rinstitute.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ASoC: tas5805m: fix pdn polarity

On Wed, Mar 09, 2022 at 09:55:40PM +0000, Mark Brown wrote:
> On Wed, Mar 09, 2022 at 08:16:07PM +0000, John Keeping wrote:
> > On Wed, Mar 09, 2022 at 04:28:30PM +0000, Mark Brown wrote:
> 
> > > I think the device tree binding needs to be clarified here to be
> > > explicit about this since there's obviously some room for user confusion
> > > here.  We can probably get away with a change at this point since it's
> > > not hit a release but we do need to try to avoid the situation where any
> > > other implementations use active high polarity for the bindings.
> 
> > Taking a quick survey of the other devices that have a pdn-gpios
> > property:
> 
> > - tvp5150 is correct with the driver setting 0 to make the device active
> 
> > - tas571x also sets 0 to make the device active
> 
> > - ak4375 uses the opposite sense setting PDN = 1 to make the device
> >   active; this has no in-tree users and was merged as part of v5.17-rc1
> >   so it's not in a released kernel yet
> 
> Sure, I still think it would be good to update the binding document to
> clarify things as part of your patch - the binding currently just has it
> as the "pdn" pin not the /pdn pin or anything.

I've been thinking about this but I can't really think what to say.
tas571x's binding says:

	GPIO specifier for the TAS571x's active low powerdown line

Is that the sort of wording you have in mind?

To me it seems like a general principle that the GPIO_ACTIVE_{HIGH,LOW}
flags should be used to indicate how the pin works so that the driver
consistently uses logical levels regardless of how the hardware is
wired.

>From the driver point of view pdn-gpios is effectively reset-gpios by
another name and it's pretty consistent that setting a reset GPIO to 1
means the device is inaccessible.

Maybe this just means I'm approaching this "down" from the software
abstraction more than "up" from the hardware.